The implementation of the European Energy Performance of Buildings Directive (EPBD 2002) requires all EU Member States to develop a procedure to quantitatively determine the energy performance of buildings. That procedure can not cover all possible kinds of new technologies, so Member States have to develop assessment procedures according to the "principle of equivalence", in order to assess those systems which are not covered by the standard calculation procedure. Without these alternative calculation methods innovating technology can not be validated by which research and development would be obstructed. In Belgium a methodology for the assessment of innovative residential ventilation systems was developed by a collaboration of the University of Ghent, the Catholic University of Leuven and the Belgian Building Research Institute. In principle, there were two major focal points: for one thing there has to be a broad consensus on the scientific reliability of the calculation methodology, on the other hand one should bear in mind that the slightest gap or flaw in methodology can cause improper use. For example if one should use only one occupancy schedule, a ventilation system with an automatic time switch designed according to that schedule would appear to have an excellent performance. Bearing that in mind the switch from a deterministic approach to a stochastic method was swiftly taken and Monte Carlo Analysis (MCA) came to the fore. Eventually the formulated methodology requires quite a lot of simulations to assess the innovative ventilation system: five degrees of airtightness are used, three systems have to be compared, and the MCA requires 100 simulations per system. In total 1500 simulations have to be done. The multizone airflow network model Contam was used because of its extensive possibilities in control options (the innovative system was humidity controlled) to perform the simulations. MCA is not a standard integrated tool in Contam so an interoperability algorithm in excel was developed to automatize the MCA process. The software interoperability is relatively complicated, and most companies do not have the means to develop complicated algorithms, nor do they have the time to do 1500 simulations while being confronted with optimization issues. Because of this a methodology with a restricted number of simulations and variables will be developed which can be used for optimization within a set confidence interval in line with the obligatory methodology. How many occupancy schedules have to be considered, which variables have a major influence, and how to deal with uncertainty? In order to avoid that the methodology becomes a barrier for innovation an engineering approach will supply in a feasible calculation and simulation method for optimization.